Traditional semiconductor integrated circuit technology is commonly used to integrate various electronic circuits onto a common semiconductor substrate to form an electronic system, or subsystem. The traditional approach to integrating circuits into a system often has process, manufacturing and design limitations which present difficulties when certain electronic circuits are integrated onto a common semiconductor substrate. A recently developed integration technology commonly referred to as system-in-package (SiP) technology attempts to overcome at least some of the limitations of traditional semiconductor integration methods by interconnecting multiple discrete and individually fabricated semiconductor systems on a common substrate and encapsulating the complete system in a common package. Accordingly, SiP allows a variety of device technologies to be integrated into a single package that would otherwise be difficult and expensive to fabricate using traditional integration methods. For example, SiP technology has been successfully applied in mixed signal applications, where analog and digital components are integrated onto the same chip. Such applications typically present noise immunity difficulties, since digital circuit switching commonly injects noise into the common substrate, which may corrupt sensitive analog signals. As the size of features in devices decreases and clock frequencies increase, the amount of substrate noise created by digital switching has increased dramatically.
As previously mentioned, the multiple discrete systems of a SiP are electrically coupled together to form a system and, as is well known in the art of digital electronics, many of the multiple systems communicate with one another by transmitting digital information in the form of electrical signals. Typically, even analog-based systems in the SiP generally have analog signals converted into the digital domain. The electrical signals transmitted between the multiple systems generally represent a serial data stream where the data is represented by binary states having discrete levels of amplitude or phase, as well known. Multiple electrical signals are transmitted in parallel to transmit data of a data width, with each signal representing one bit of the width of data. In transmitting the data, the electrical signal may be distorted by various phenomena, such as noise, signal strength variations, phase shift variations, and the like. Moreover, multiple individual devices generally interact in a SiP, and the various devices may operate at different voltage levels that may cause undesired electrical currents to flow from one system to another, which generally contributes to excess power consumption. Additionally, the undesired current may be sufficiently large to damage to the devices.
Consequently, SiP devices have employed capacitively coupled signaling between the multiple systems to filter noise from the electrical signals and also prevent current flow between devices operating in different voltage domains. FIG. 1 illustrates a capacitively coupled signaling system having a capacitively coupled data bus 100 that is n-bits wide that may be used to transmit data signals D_OUT0-D_OUTn. The data bus 100 includes output driver circuits, or transmitters 102 of the transmitting device capacitively coupled through capacitors 106 to input buffer circuits, or receivers 104 at the receiving device. The received data has been represented by the received data signals D_IN0-DINn. As shown in FIG. 1, the data bus 100 has been illustrated as a unidirectional data bus, with the transmitters 102 representing a transmitting device and the receivers 104 representing a receiving device. However, it will be appreciated that the data bus 100 has been illustrated in this manner by way of example, and that the data bus 100 can be a bi-directional data bus as well.
Lower power may be consumed when utilizing capacitively coupled signaling since there is only minimal leakage current between devices. Capacitively coupled signaling is also insensitive to voltage domains, allowing operation without the need for level shifting. Specifically, a capacitively coupled signaling system permits an AC component of a signal to be transferred, while blocking a DC component of the signal. Additionally, circuits designed for protection from electrostatic discharge (ESD) are no longer necessary where the signaling is entirely contained within the SiP device. Circuits dedicated to ESD protection, usually consisting of diode networks in various configurations, add complexity to the terminal regions of a device, and compete for “real estate” on the device substrate. Load requirements on output circuitry can also be relaxed compared with conventional off-die signaling because the need to drive signals external to the device package are eliminated for those signals that remain internal to the SiP device.
In forming capacitively coupled signaling systems, discrete passive components have been used to connect the signal terminals of the different systems, such as discrete capacitors, resistors, and the like. However, when discrete components are used, some of the foregoing advantages associated with a capacitively coupled signaling system are reduced. For example, when a signal pad is wire bonded to a discrete passive component that further extends to another signal pad, parasitic effects are generally introduced. Additionally, when several discrete components are included in a SiP, an increased form factor is generally developed, since the additional components must be accommodated. Passive components can be integrated into each discrete system, thereby avoiding issues with having additional passive components included in the SiP, but even when the passive components are integrated into the SiP, the need to have wires coupling the signal pads of the discrete systems cannot be avoided. As noted above, bonding wires can cause undesirable parasitic loading effects. Therefore, there is a need in the art for an alternative capacitively coupled signaling structure and a method for forming a capacitively coupled structure.